1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP MR sensor with an improved seed layer structure for the hard bias layer that longitudinally biases the sensor free layer.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP MR sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-GMR read head the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
The sensor stack in a CPP MR read head is located between two shields of magnetically permeable material that shield the read head from recorded data bits on the disk that are neighboring the data bit being read. The sensor stack has an edge that faces the disk with a width referred to as the track width (TW). The sensor stack has a back edge recessed from the edge that faces the disk, with the dimension from the disk-facing edge to the back edge referred to as the stripe height (SH). The sensor stack is generally surrounded at the TW edges and back edge by insulating material.
A layer of hard or high-coercivity ferromagnetic material, typically a CoPt or CoPtCr alloy, is used as a “hard bias” layer to stabilize the magnetization of the free layer longitudinally via magneto-static coupling. The hard bias layer is deposited as an abutting junction onto insulating material on each side of the TW edges of the sensor. The hard bias layer is required to exhibit a generally in-plane magnetization direction with high coercivity (Hc) to provide a stable longitudinal bias that maintains a single domain state in the free layer so that the free layer will be stable against all reasonable perturbations while the sensor maintains relatively high signal sensitivity. The hard bias layer must have sufficient in-plane remanent magnetization (Mr), which may also be expressed as Mrt since Mr is dependent on the thickness (t) of the hard bias layer. Mrt is the component that provides the longitudinal bias flux to the free layer and must be high enough to assure a single magnetic domain in the free layer but not so high as to prevent the magnetic field in the free layer from rotating under the influence of the magnetic fields from the recorded data bits. Moreover, a high squareness (S) hard bias material is desired, i.e., S=Mr/Ms should approach 1.0, where Ms is the saturation magnetization.
The desired magnetic properties of the CoPt or CoPtCr alloy hard bias layer are typically achieved by a seed layer or layers directly below the hard bias layer. Various types of seed layers, including CrMo, CrTi and TiW alloys, and bilayers, including NiTa/CrMo and CrMo/W bilayers, have been proposed in the prior art. In addition to achieving the desired magnetic properties for the subsequently grown CoPt or CoCrPt alloy, the seed layer should also be as thin as possible. This is because as the data density increases in magnetic recording disk drives, there is a requirement for a decrease in the read head dimensions, particularly the shield-to-shield spacing. However, because it is desirable to maintain the thickness of the hard bias layer to assure magnetic stabilization of the free layer, the thickness of the seed layer has to be reduced accordingly.
What is needed is a CPP MR sensor with an improved seed layer structure that can be made very thin yet still provide desirable magnetic properties for the hard bias layer.